1. Field of the Invention
The present invention relates to a receptacle-type LD module.
2. Description of the Background Art
A receptacle-type optical module is a module that contains an LD or a PD in its package and that allows the attaching and detaching of a ferrule to which an optical fiber is attached. Furthermore, the module has a feature in which the plane of the package is orthogonal to the optical axis of the fiber, which is known as the perpendicular coupling. The module has so far been used widely. an LD module is provided with a laser diode (LD) in it. They have a receptacle-type structure in which a ferrule having an attached optical fiber can be connected to or disconnected from them. If light emitted from the laser diode is reflected to return to it, the operation of the laser diode becomes unstable, which is undesirable. in the case of the receptacle type, because the ferrule having an attached optical fiber is attached to or detached from the receptacle, the end portion of the optical fiber cannot be cut obliquely. The opposing faces are required to be perpendicular to the optical axis of the optical fiber. To solve the problem, a design is employed in which the receptacle is provided in it with a dummy fiber whose end is cut obliquely so that the light emitted from the LD can be obliquely reflected from the oblique end of the dummy fiber to prevent the light from returning to the laser diode. Such a design concept is employed in many types of receptacles, one of which has been disclosed in the patent literature 1, which is the published Japanese patent application Tokukai 2003-241025 entitled “Optical receptacle and optical module using it.”
As shown in FIG. 5, first, a sleeve 79 is inserted into a hole 78 of a top part of the receptacle 78. A stub 77 is prepared into which a dummy fiber 87 is inserted and the rear end of which is obliquely polished. The stub 77 is press-fitted into the bottom part of the receptacle 85. And then the top part of the receptacle 78 is press-fitted into the bottom part of the receptacle 85. The stub 77 is in contact with the receptacle 73 and the sleeve 79 through a contacting surface 76. A holder 62 has at its rear portion an optical-device-housing space 80 into which an LD (optical device) 75 is inserted to be fixed there. The holder 62 is provided at its middle portion with a lens-holding wall and a through hole 83. A spherical lens 74 is fixed at the front of the through hole 83. The LD 75 is placed at the rear of the through hole 83, so that the light of the LD passes through an inner space 86 to enter the dummy fiber 87. At this moment, the light reflected from the dummy fiber 87 is reflected at an oblique angle of 2Θ, which is two times the obliquely cut angle, Θ, of the stub 77. Consequently, it does not return to the laser diode 75. In this design, a ferrule 93 into which an outside optical fiber 94 is inserted can be attached to or detached from the receptacle 73. A front-end portion 88 of the receptacle 73 can be fitted into an internal space 95 of an outer cylindrical portion 92 of the ferrule 93 and be securely held there. The light having entered the dummy fiber 87 from the laser diode 75 enters the outside optical fiber 94 to be transmitted to the outside.
The above-described method in which an obliquely cut fiber prevents the backreflection light from returning to the laser diode is a conventional practice.
The mechanism for preventing the backreflection by utilizing the oblique reflection from the obliquely cut optical fiber is simple in structure and advantageous in that not many additional components are required. A number of proposals have been made for the oblique cutting. Some of them are put into actual use. Nevertheless, this technique still has some problems.
First, high dimensional precision is required because the obliquely cut stub 77 having the dummy fiber 87 is press-fitted into the hole 78 of the receptacle 73 and fixed there. It is necessary that the center of the sleeve 79 be coincident with that of the dummy fiber 87. Extremely high finishing precision is required for the hole 78 of the receptacle 73 and for the outer circumference of the sleeve 79. It seems that the hole 78 can be widened for the stub 77 to be fixed with an adhesive. However, this method is not desirable because the dummy fiber 87 tends to be displaced form the center of the axis.
Finishing work is required for the oblique cutting and polishing work of the end face of the stub 77. This additional process increases the part cost of the stub.
In addition, because the stub 77 into which the dummy fiber 87 is inserted is to be axially inserted into the receptacle 73, the longitudinal dimension of the receptacle 73 becomes large. This causes a problem of difficulty in miniaturization. Because the optical axis of the dummy fiber must be coincident with that of the attached fiber and the LD, the stub is required to have a length of a certain extent (about 2 mm or more) in order to secure the parallelism. Therefore, its longitudinal dimension cannot be decreased.
An LD has anisotropy in light emission. Consequently, if the direction of the oblique cutting is not coincident with that of the LD, the coupling efficiency varies. Therefore, it is necessary to assemble them while the intensity of the output light (efficiency) is being measured. This poses a problem in that the assembly work of the module becomes difficult.
Furthermore, the attaching and detaching of the optical fiber sometimes varies the coupling efficiency of the optical fiber, unstabilizing the attaching-and-detaching property.